Predicted elements of telomere organization and function in Ustilago maydis

Telomeres are specialized caps of nucleoprotein complexes located at the chromosome termini. They consist of short DNA repeats and of an assortment of associated proteins whose function is currently under intense investigation in model systems. These specialized structures protect the linear ends of eukaryotic chromosomes against DNA repair and degradation activities, and serve as the substrate for telomerase, the ribonucleoprotein complex that synthesises the telomere repeats. The pivotal role of the telomeres in the maintenance of cell viability in several model eukaryotes, including humans, greatly promoted research in telomere biology. Studies on telomere structure and function in fungi other than model systems are limited to providing information on the telomeric repeat sequences. Here, we have summarized the current knowledge on the organization of chromosome ends and on the proteins participating in telomere function in model systems including recent information obtained for filamentous fungi. We also describe Ustilago maydis genes that are potential homologs of proteins known from other systems to participate in telomere biology.     

Function, replication and structure of the mammalian telomere

Telomeres are specialized structures at the ends of linear chromosomes that were originally defined functionally based on observations first by Muller (1938) and subsequently by McClintock (1941) that naturally occurring chromosome ends do not behave as double-stranded DNA breaks, in spite of the fact that they are the physical end of a linear, duplex DNA molecule. Double-stranded DNA breaks are highly unstable entities, being susceptible to nucleolytic attack and giving rise to chromosome rearrangements through end-to-end fusions and recombination events. In contrast, telomeres confer stability upon chromosome termini, as evidenced by the fact that chromosomes are extraordinarily stable through multiple cell divisions and even across evolutionary time. This protective function of telomeres is due to the formation of a nucleoprotein complex that sequesters the end of the DNA molecule, rendering it inaccessible to nucleases and recombinases as well as preventing the telomere from activating the DNA damage checkpoint pathways. The capacity of a functional end-protective complex to form is dependent upon maintenance of sufficient telomeric DNA. We have learned a great deal about telomere structure and how this specialized nucleoprotein complex confers stability on chromosome ends since the original observations that defined telomeres were made. This review summarizes our current understanding of mammalian telomere replication, structure and function.     

Mice with bad ends: mouse models for the study of telomeres and telomerase in cancer and aging

Telomeres are capping structures at the ends of eukaryotic chromosomes, which consist of repetitive DNA bound to an array of specialized proteins. Telomeres are part of the constitutive heterochromatin and are subjected to epigenetic modifications. The function of telomeres is to prevent chromosome ends from being detected as damaged DNA. Both the length of telomere repeats and the integrity of the telomere-binding proteins are important for telomere protection. Telomere length is regulated by telomerase, by the telomere-binding proteins, as well as by activities that modify the state of the chromatin. Various mouse models with altered levels of telomerase activity, or mutant for different telomere-binding proteins, have been recently generated. Here, I will discuss how these different mouse models have contributed to our understanding on the role of telomeres and telomerase in cancer and aging.     

Manipulating mouse telomeres: models of tumorigenesis and aging

Telomeres are capping structures at the ends of chromosomes, composed of a repetitive DNA sequence and associated proteins. Both a minimal length of telomeric repeats and telomere-associated binding proteins are necessary for proper telomere function. Functional telomeres are essential for maintaining the integrity and stability of eukaryotic genomes. The capping structure enables cells to distinguish chromosome ends from double strand breaks (DSBs) in the genome. Uncapped chromosome ends are at great risk for degradation, recombination, or chromosome fusion by cellular DNA repair systems. Dysfunctional telomeres have been proposed to contribute to tumorigenesis and some aging phenotypes. The analysis of mice deficient in telomerase activity and other telomere-associated proteins has allowed the roles of dysfunctional telomeres in tumorigenesis and aging to be directly tested. Here we will focus on the analysis of different mouse models disrupted for proteins that are important for telomere functions and discuss known and proposed consequences of telomere dysfunction in tumorigenesis and aging.     

All's well that ends well.

A recent Cold Spring Harbour meeting(*) reviewed the latest progress on telomeres (the specialized structures that form the ends of chromosomes) and telomerase (the enzyme primarily responsible for their replication). Among the many aspects of telomere biology covered were strong sessions elaborating telomere replication and length regulation, telomerase structure and function, end-binding and telomere-associated proteins, DNA-damage-response proteins and telomerase-independent telomere maintenance.     

Interaction of an Arabidopsis RNA-binding protein with plant single-stranded telomeric DNA modulates telomerase activity

Telomeres are the specialized structures at the end of linear chromosomes and terminate with a single-stranded 3' overhang of the G-rich strand. The primary role of telomeres is to protect chromosome ends from recombination and fusion and from being recognized as broken DNA ends. This protective function can be achieved through association with specific telomere-binding proteins. Although proteins that bind single-stranded G-rich overhang regulate telomere length and telomerase activity in mammals and lower eukaryotes, equivalent factors have yet to be identified in plants. Here we have identified proteins capable of interacting with the G-rich single-stranded telomeric repeat from the Arabidopsis extracts by affinity chromatography. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis indicates that the isolated protein is a chloroplast RNA-binding protein (and a truncated derivative). The truncated derivative, which we refer to as STEP1 (single-stranded telomere-binding protein 1), binds specifically the single-stranded G-rich plant telomeric DNA sequences but not double-stranded telomeric DNA. Unlike the chloroplast-localized full-length RNA-binding protein, STEP1 localizes exclusively to the nucleus, suggesting that it plays a role in plant telomere biogenesis. We also demonstrated that the specific binding of STEP1 to single-stranded telomeric DNA inhibits telomerase-mediated telomere extension. The evidence presented here suggests that STEP1 is a telomere-end binding protein that may contribute to telomere length regulation by capping the ends of chromosomes and thereby repressing telomerase activity in plants.     

Telomere dysfunction in aging and cancer

Telomeres are unique DNA-protein structures that contain noncoding TTAGGG repeats and telomere-associated proteins. These specialized structures are essential for maintaining genomic integrity. Alterations that lead to the disruption of telomere maintenance result in chromosome end-to-end fusions and/or ends being recognized as double-strand breaks. A large body of evidence suggests that the cell responds to dysfunctional telomeres by undergoing senescence, apoptosis, or genomic instability. In conjunction with other predisposing mechanisms, the genomic instability encountered in preimmortal cells due to dysfunctional or uncapped telomeres might lead to cancer. Furthermore, telomere dysfunction has been proposed to play critical roles in aging as well as cancer progression. Conversely, recent evidence has shown that targeting telomere maintenance mechanisms and inducing telomere dysfunction in cancer cells by inhibiting telomerase can lead to catastrophic events including rapid cell death and increased sensitivity to other cancer therapeutics. Thus, given the major role telomeres play during development, it is important to continue our understanding telomere structure, function and maintenance. Herein, we provide an overview of the emerging knowledge of telomere dysfunction and how it relates to possible links between aging and cancer.     

哺乳动物早期胚胎端粒和端粒酶重编程

端粒位于真核染色体末端,是稳定染色体末端的重要元件。端粒酶(TER)是一种特殊的细胞核糖核蛋白(RNP)反转录酶(RT),其核心酶包括蛋白亚基和RNA元件。在DNA复制过程中的端粒丢失可以被有活性的端粒酶修复回来。哺乳动物端粒酶在发育中受调控,端粒的重编程可能是由于早期胚胎不同时期的端粒酶活性而造成的。因此,研究端粒和端粒酶重编程在早期胚胎发育中是非常重要的。该文综述了端粒和端粒酶的结构和功能,及其与哺乳动物早期胚胎发育的关系,并在此基础上展望了端粒和端粒酶在克隆动物胚胎发育的基础研究。     

Telomere length, chromatin structure and chromosome fusigenic potential

Telomeres are specialized structures at chromosome ends that are thought to function as buffers against chromosome fusion. Several studies suggest that telomere shortening may render chromosomes fusigenic. We used a novel quantitative fluorescence in situ hybridization procedure to estimate telomere length in individual mammalian chromosomes, and G-banding and chromosome painting techniques to determine chromosome fusigenic potential. All analysed Chinese hamster and mouse cell lines exhibited shorter telomeres at short chromosome arms than at long chromosome arms. However, no clear link between short telomeres and chromosome fusigenic potential was observed, i.e. frequencies of telomeric associations were higher in cell lines exhibiting longer telomeres. We speculate that chromosome fusigenic potential in mammalian cell lines may be determined not only by telomere length but also by the status of telomere chromatin structure. This is supported by the observed presence of chromatin filaments linking telomeres in Chinese hamster chromosomes and of multibranched chromosomes oriented end-to-end in the murine severe combined immunodeficient (SCID) cell line. Multibranched chromosomes are the hallmark of the human ICF (Immune deficiency, Centromeric instability, Facial abnormalities) syndrome, characterized by alterations in heterochromatin structure. Received: 13 June 1997; in revised form: 3 August 1997 / Accepted: 4 August 1997     

The role of DNA damage response proteins at telomeres--an "integrative" model

Telomeres are specialized structures at chromosome ends which play the key role in chromosomal end protection. There is increasing evidence that many DNA damage response proteins are involved in telomere maintenance. For example, cells defective in DNA double strand break repair proteins including Ku, DNA-PKcs, RAD51D and the MRN (MRE11/RAD51/NBS1) complex show loss of telomere capping function. Similarly, mouse and human cells defective in ataxia telangiectasia mutated (ATM) have defective telomeres. A total of 14 mammalian DNA damage response proteins have, so far, been implicated in telomere maintenance. Recent studies indicate that three more proteins, namely BRCA1, hRad9 and PARP1 are involved in telomere maintenance. The involvement of a wide range of DNA damage response proteins at telomeres raises an important question: do telomere maintenance mechanisms constitute an integral part of DNA damage response machinery? A model termed the "integrative" model is proposed here to argue in favour of telomere maintenance being an integral part of DNA damage response. The "integrative" model is supported by the observation that a telomeric protein, TRF2, is not confined to its local telomeric environment but it migrates to the sites of DNA breakage following exposure of cells to ionizing radiation. Furthermore, even if telomeres are maintained in a non-canonical way, as in the case of Drosophila, DNA damage response proteins are still involved in telomere maintenance suggesting integration of telomere maintenance mechanisms into the DNA damage response network.     

Telomeres and the DNA damage response: why the fox is guarding the henhouse

DNA double strand breaks (DSBs) are repaired by an extensive network of proteins that recognize damaged DNA and catalyze its repair. By virtue of their similarity, the normal ends of linear chromosomes and internal DNA DSBs are both potential substrates for DSB repair enzymes. Thus, telomeres, specialized nucleo-protein complexes that cap chromosomal ends, serve a critical function to differentiate themselves from internal DNA strand breaks, and as a result prevent genomic instability that can result from their inappropriate involvement in repair reactions. Telomeres that become critically short due to failure of telomere maintenance mechanisms, or which become dysfunctional by loss of telomere binding proteins, elicit extensive checkpoint responses that in normal cells blocks proliferation. In this situation, the DNA DSB repair machinery plays a major role in responding to these "damaged" telomeres - creating chromosome fusions or capturing telomeres from other chromosomes in an effort to rid the cell of the perceived damage. However, a surprising aspect of telomere maintenance is that many of the same proteins that facilitate this repair of damaged telomeres are also necessary for their proper integrity. Here, we review recent work defining the roles for DSB repair machinery in telomere maintenance and in response to telomere dysfunction.     

Telomere dysfunction in genome instability syndromes

Telomeres are nucleoprotein complexes located at the end of eukaryotic chromosomes. They have essential roles in preventing terminal fusions, protecting chromosome ends from degradation, and in chromosome positioning in the nucleus. These terminal structures consist of a tandemly repeated DNA sequence (TTAGGG in vertebrates) that varies in length from 5 to 15 kb in humans. Several proteins are attached to this telomeric DNA, some of which are also involved in different DNA damage response pathways, including Ku80, Mre11, NBS and BLM, among others. Mutations in the genes encoding these proteins cause a number of rare genetic syndromes characterized by chromosome and/or genetic instability and cancer predisposition. Deletions or mutations in any of these genes may also cause a telomere defect resulting in accelerated telomere shortening, lack of end-capping function, and/or end-to-end chromosome fusions. This telomere phenotype is also known to promote chromosomal instability and carcinogenesis. Therefore, it is essential to understand the interplay between telomere biology and genome stability. This review is focused in the dual role of chromosome fragility proteins in telomere maintenance.     

Structure of telomeric chromatin in <Emphasis Type="Italic">Drosophila</Emphasis>

The telomeric nucleoprotein complex protects linear chromosome ends from degradation. In contrast to most eukaryotes in which telomerase is responsible for telomere elongation by adding short DNA repeats synthesized using an RNA template, the telomere elongation in Drosophila involves transposition of specialized telomeric retroelements onto chromosome ends. Proteins that bind telomeric and subtelomeric sequences form specific telomeric chromatin, and its components are highly conserved among organisms employing different mechanisms of telomere elongation. This review is focused on the analysis of components of the Drosophila telomeric complex and its comparison with telomeric proteins in telomerase-encoded organisms. Structural and functional analysis of Drosophila telomeres suggests that there are three distinct chromatin regions: protective structure at the very end of chromosome (cap), subtelomeric region which is characterized by condensed chromatin structure, and the terminal retrotransposon array whose expression is under the control of an RNAi (RNA interference)-based mechanism. The link between RNAi and telomeric chromatin formation in germinal tissues is discussed.     

Epigenetic maintenance of telomere identity in Drosophila: Buckle up for the sperm ride

A critical function of telomeres is to prevent the ligation of chromosome ends by DNA repair enzymes. In most eukaryotes, telomeric DNA consists in large arrays of G-rich tandem repeats that are recognized by DNA binding capping proteins. Drosophila telomeres are unusual as they lack short tandem repeats. However, Drosophila capping proteins can bind chromosome extremities in a DNA sequence-independent manner. This epigenetic protection of fly telomeres has been essentially studied in somatic cells where capping proteins such as HOAP or HP1 are essential in preventing chromosome end-to-end fusions. HipHop and K81 are two recently identified paralogous capping proteins with complementary expression patterns. While HipHop is involved in telomere capping in somatic cells, K81 has specialized in the protection of telomeres in post-meiotic male germ cells. Remarkably, K81 is required for the stabilization of HOAP and HP1 at telomeres during the massive paternal chromatin remodeling that occurs during spermiogenesis and at fertilization. We thus propose that the maintenance of capping proteins at Drosophila sperm telomeres is crucial for the transmission of telomere identity to the diploid zygote.     

Telomeres, chromosome instability and cancer

Telomeres are composed of repetitive G-rich sequence and an abundance of associated proteins that together form a dynamic cap that protects chromosome ends and allows them to be distinguished from deleterious DSBs. Telomere-associated proteins also function to regulate telomerase, the ribonucleoprtotein responsible for addition of the species-specific terminal repeat sequence. Loss of telomere function is an important mechanism for the chromosome instability commonly found in cancer. Dysfunctional telomeres can result either from alterations in the telomere-associated proteins required for end-capping function, or from alterations that promote the gradual or sudden loss of sufficient repeat sequence necessary to maintain proper telomere structure. Regardless of the mechanism, loss of telomere function can result in sister chromatid fusion and prolonged breakage/fusion/bridge (B/F/B) cycles, leading to extensive DNA amplification and large terminal deletions. B/F/B cycles terminate primarily when the unstable chromosome acquires a new telomere, most often by translocation of the ends of other chromosomes, thereby providing a mechanism for transfer of instability from one chromosome to another. Thus, the loss of a single telomere can result in on-going instability, affect multiple chromosomes, and generate many of the types of rearrangements commonly associated with human cancer.     

端粒、端粒酶结构功能研究进展

端粒是真核生物线性染色体末端由重复DNA序列和蛋白质结合形成的复合结构,其特殊的环形结构与多种结合蛋白形成了端粒的多重功能的基础。端粒的功能包括染色体末端的保护、引导减数分裂的同源染色体配对、参与DNA修复过程等;端粒酶具有逆转录酶特性和维持端粒长度的功能,其活性与恶性肿瘤的发生密切相关,调控因子错综复杂。     

TRF2 Protein Interacts with Core Histones to Stabilize Chromosome Ends

Mammalian chromosome ends are protected by a specialized nucleoprotein complex called telomeres. Both shelterin, a telomere-specific multi-protein complex, and higher order telomeric chromatin structures combine to stabilize the chromosome ends. Here, we showed that TRF2, a component of shelterin, binds to core histones to protect chromosome ends from inappropriate DNA damage response and loss of telomeric DNA. The N-terminal Gly/Arg-rich domain (GAR domain) of TRF2 directly binds to the globular domain of core histones. The conserved arginine residues in the GAR domain of TRF2 are required for this interaction. A TRF2 mutant with these arginine residues substituted by alanine lost the ability to protect telomeres and induced rapid telomere shortening caused by the cleavage of a loop structure of the telomeric chromatin. These findings showed a previously unnoticed interaction between the shelterin complex and nucleosomal histones to stabilize the chromosome ends.